Its results breezed past analysts’ expectations, and Aetna shares jumped more than 5 percent Thursday after it detailed its quarterly performance.

The Hartford, Conn., insurer closed a $6.9 billion acquisition of fellow insurer Coventry Health Care last May, and it said Thursday that deal was the main factor behind its growth in this year’s first quarter.

Aetna Inc. is the nation’s third-largest health insurer, and its medical enrollment swelled 24 percent in the quarter to 22.7 million people versus last year.

Coventry serves customers in two markets primed for growth. It administers Medicaid, the state and federally funded program that covers the needy and disabled people, and it offers Medicare Advantage plans. Those are subsidized versions of the federal government’s Medicare program for the elderly and also disabled people.

Aetna Chairman and CEO Mark Bertolini told analysts on Thursday that the insurer added about 130,000 Medicare Advantage customers during the quarter.

It also added another 230,000 paying customers through public insurance exchanges that debuted this year, courtesy of the health care overhaul. The federal law set up state-based exchanges on which customers can shop for coverage with help from income-based tax credits.

Bertolini said Aetna expects to add about 450,000 paying customers this year through the exchanges, and the risk of that business appears to be manageable so far, although he cautioned that they still don’t have a good sense for what types of claims these customers will generate.

Insurers and analysts have been worried that sick people who have been unable to find decent coverage would overload these exchanges at first, which could strain insurer balance sheets and lead to future price hikes.

Overall, Aetna earned $665.5 million, or $1.82 per share, in the quarter that ended March 31. That’s up from $490.1 million, or $1.48 per share, a year earlier.

Adjusted earnings totaled $1.98 per share, not counting one-time items like a $92 million loss from the early retirement of some long-term debt and costs tied to the Coventry deal.